The present invention relates generally to current limiting and fault indicating circuits and more specifically to circuits with dual current limiting devices such as tungsten lamps and transistors in lieu of circuit breakers for currents ranging from fractions of an amphere to about three ampheres.
Electrical circuits have generally included some form of protection against the unrestricted flow of current resulting from the occurrence of a short circuit. Oftentimes, this protection takes the form of a common fuse which completely interrupts the flow of current in the event of a short circuit. The fuse must then be replaced to restore current in the electrical circuit. Circuit breakers may also provide protection for electrical wiring against current overloading. However, circuit breakers are generally electro-mechanical or electro-magnetic devices which must be reset to restore the flow of current in the circuit. In the event of a short circuit, a circuit breaker may "hang up" and fail to open the circuit due to a welding of the contacts or mechanical binding of the contact mechanism and thus cause a smoke or fire hazard.
Circuits including two or more lamp filaments connected in parallel have been useful for indicating short circuits in low power systems. A circuit of this type, for example, is disclosed in U.S. Pat. No. 3,074,013 issued to R. N. Bowden. The lamp filaments in these circuits simultaneously indicate the existence of a short circuit and limit the current that may flow through the short circuit. In the event of a short circuit, however, these circuits do not limit the current that may flow through the circuit below the level of current flow permitted by the lamps in the absence of a short circuit.
Accordingly, it is an object of the present invention to provide a novel method and circuit having a pair of current limiting devices connected in parallel to both indicate the existence of an overcurrent fault and to limit the current that may flow through a short circuit to a level below the level of current flow permitted by the circuit in the absence of a short circuit.
Another object of the present invention is to provide a novel method and current limiting and fault indicating circuit having a high voltage-low current device and a low voltage-high current device connected in parallel in which the low voltage-high current device is automatically removed from the circuit in the event of a short circuit.
Still another object of the present invention is to provide a novel method and current limiting and fault indicating circuit having a remote status indication of the existence of a short circuit.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from the claims and from the perusal of the following detailed description in connection with the appended drawings.